This invention relates generally to the laying of off-shore pipelines and more particularly concerns an assemblage and method for undersea laying of pipeline from a ship spool.
In present practice, undersea pipelines are often laid in lengths which have been reeled onto a ship spool at radii within the plastic deformation range of the pipe. The pipeline is unreeled from the spool to a tower which is pivoted at or above deck level for rotation of the upper end of the tower toward or away from the spool. The shallower the water in which pipeline is laid, the shallower the angle of inclination of the tower. At 20 meter water depths the tower approaches horizontal, while above 200 meter water depths the tower approaches vertical. Depths approximating 2000 meters are not uncommon.
The tower supports the components of the unreeling system including, in sequential order from the top of the tower, at least an aligner, a straightener and a tensioner, all above a work station located in the tower, and a holding clamp beneath the exit level of the pipeline from the tower to suspend the weight of the exited pipeline during work on pipeline in the tower above. For use in shallower depth applications, the distance from the tower pivot to the axis of rotation of the spool must be greater than the sum of the height of the tower plus the radius of the spool. The ship must be designed to accommodate this distance. The ship must also be designed to accommodate the weight and dimensions of the reeled pipeline and the tower and to compensate for the variations in buoyancy requirements resulting from the orientation of the tower and the weight of the unreeled pipeline suspended from the tower, which could be anywhere from 20 to 2000 meters in length. Moreover, all of these dimensional difficulties are compounded by the plasticity of the pipeline over the distance from the spool to the straightener which, depending on the orientation of the tower, ranges from more than half to more than the total height of the tower.
Bottom line, such a ship typically approximates 330 feet in length and, including the lay equipment on the ship, often leases for as much as $500,000 per day. It is, therefore, an object of this invention to provide a method and assemblage for reel laying off-shore pipelines which require smaller and/or less expensive ships than conventional pipe lay systems. It is also an object of this invention to provide a method and assemblage for laying off-shore pipelines which require less shipboard space than conventional pipe lay systems. A further object of this invention is to provide a method and assemblage for laying off-shore pipelines which employ fewer components than conventional pipe lay systems.